1. Field of the Invention
This invention relates to gypsum-based construction materials. More particularly, the invention relates to a gypsum-based substrate faced with a synthetic polymeric sheet material that is suited for use as a construction material such as wallboard or ceiling panels. The invention is also directed to a process for manufacturing a gypsum-based substrate faced with a synthetic polymeric sheet material.
2. Description of Related Art
Gypsum wallboard is traditionally manufactured by a continuous process. In the process, a gypsum slurry is first generated in a mechanical mixer by mixing calcium sulphate hemihydrate (also known as calcined gypsum), water, and other agents. The gypsum slurry is deposited on a paper sheet. An upper continuously advancing paper sheet is laid over the gypsum and the edges of the upper and lower sheets are pasted to each other. The paper sheets and gypsum slurry are passed between parallel upper and lower forming plates or rolls in order to generate an integrated and continuous flat strip of unset gypsum sandwiched between the paper sheets that are known as facing or liners. This strip is conveyed over a series of continuous moving belts and rollers for a period of 2 to 5 minutes during which time the core begins to hydrate back to gypsum and hardens. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the paper facing to delaminate from the gypsum core if the adhesion between the gypsum core and the facing is not sufficient. Once the gypsum core has set sufficiently, the continuous strip is cut into shorter lengths or even individual boards or panels of prescribed length. Once again, it is important for there to be good adhesion between the paper sheets and the set, but still wet, gypsum core or the cutting action will pull the edges of the paper facing sheet away from the gypsum core.
After the cutting step, the gypsum boards are separated and grouped through a series of belts and rollers and then flipped over before being fed into drying ovens or kilns where the boards are dried so as to evaporate excess water. The hydration from hemihydrate to gypsum must be essentially complete by this point, normally between 7 and 15 minutes after mixing. When the gypsum boards are accelerated, flipped and fed into the drying ovens, the boards are subjected to a variety of stresses that can cause the facing to peel away from the gypsum core of the boards unless there is good adhesion between the set (but still wet) gypsum core and the facing material. Inside the drying ovens, the boards are blown with hot drying air at speeds up to 4000 feet/minute which can cause further delamination of the paper facing if there is not good wet adhesion between the gypsum and the paper liners. When portions of the facing sheets delaminate from the gypsum core during drying in the oven, the liner becomes entangled in the rollers and the gypsum crumbles as it dries, jamming the oven, which requires frequent shut downs of the line while the loose gypsum is cleaned out of the ovens. The gypsum boards are dried in the ovens for anywhere from 30 to 75 minutes. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes.
The fully dried gypsum adheres well to the paper facing sheet materials as long as the gypsum board is kept dry. However, paper facing has a number of inherent properties that can be detrimental in a gypsum wallboard product. Paper facing material (sometimes called a paper liner) is not as strong or abrasion resistant as needed for certain construction applications. In addition, because the strength of paper differs significantly depending on the direction in which the strength is measured, paper facing must be relatively thick in order to achieve satisfactory multidirectional strength. Paper faced gypsum-board products also suffer from a lack of abrasion resistance. Paper facing used on conventional gypsum board becomes especially weak and subject to delamination from the gypsum core when the paper becomes damp due to leaks or high humidity. Paper-faced gypsum boards must generally be coated with another material, such as paint or wallcoverings, in order to achieve sufficient abrasion resistance. To overcome these durability problems, paperfaced wallboard is frequently covered with a hard sheet or plastic film when used in high traffic areas.
There have been attempts to substitute stronger and more durable synthetic sheet materials for the paper liners found in conventional gypsum board products. Plastic films do not work as liners because they are not permeable to moisture which makes it difficult to remove excess water from the gypsum. Canadian Patent No. 1,189,434 discloses gypsum panels made with a facing of a moisture vapor permeable spunbonded nonwoven material. Canadian Patent No. 1,189,434 discloses gypsum panels faced with Tyvek(copyright) spunbonded olefin sheet material. Tyvek(copyright) is a registered trademark of E.I. du Pont de Nemours and Company of Wilmington, Del. Tyvek(copyright) sheets are made by solution flash-spinning polyethylene to form fine plexifilamentary fibril structures that can be thermally bonded to form sheet material. U.S. Pat. No. 5,704,179 also mentions that nonwoven mats of synthetic resin fibers can be used to face gypsum panels.
Although it has been possible to produce gypsum boards faced with synthetic fibrous sheet materials on a small laboratory scale, it has not been feasible to produce gypsum boards faced with such synthetic fibrous sheets on a commercial scale. This is because the adhesive strength between conventional fibrous synthetic sheets and the wet gypsum core (known as wet adhesion) is low. Thus, the facing peels away from the gypsum core during various points in the production process before the boards are fully dried in the drying ovens. There is a need for a process by which gypsum board faced with a synthetic fibrous sheet can be made without having portions of the facing peel away from the gypsum core. There is also a need for a gypsum board faced with a synthetic sheet that is structured such that it can be produced on a commercial scale without portions of the facing becoming delaminated from the gypsum core.
The present invention is directed to a process for manufacturing a gypsum-based substrate. The process comprises the steps of: adding calcined gypsum and water to a mixer; mixing the gypsum and water in the mixer to produce a gypsum slurry that is comprised of 50 to 65 weight percent gypsum, and is comprised of 35 to 50 weight percent of water, based on the total weight of the of the gypsum slurry, said gypsum slurry having a density in the range of 0.95 to 1.5 g/cm3; providing a first synthetic polymeric fibrous sheet, the first sheet having a first surface; pouring the gypsum slurry from said mixer onto the first surface of said first sheet and spreading the gypsum slurry over said first surface of said first sheet; providing a second fibrous sheet, the second sheet having a first surface; placing the first surface of the second sheets over the gypsum slurry that has been spread over the first surface of the first sheet; enclosing the gypsum slurry between the first and second sheets to bring the slurry into intimate contact with the first and second sheets and form an elongated strip of gypsum slurry sandwiched between the first and second sheets; allowing the elongated strip of gypsum slurry to set up and harden to form a stiff elongated strip having a solid, wet gypsum core sandwiched between the first and second sheets, the the first and second sheets adhering to the wet gypsum core with an adhesive strength of at least 7.5 lb; cutting the stiff elongated strip into gypsum board of desired length; and drying the gypsum board in a dryer to remove excess water from the gypsum boards.
Preferably, the in the step of allowing the elongated strip of gypsum slurry to set up and harden to form a stiff elongated strip having a solid, wet gypsum core sandwiched between the first and second sheets, the first and second sheets adhere to said wet gypsum core with an adhesive strength of at least 10 lb. In the most preferred process of the invention, during the step of enclosing the gypsum slurry between the first and second sheets, the first and second sheets move at a linear speed of at least 50 ft/min.
According to the preferred process of the invention, the second sheet is a synthetic polymeric fibrous sheet. The first surface of the first sheet and the first surface of the second sheet have open pores between fibers of sufficient size for the gypsum slurry to enter the pores and become intertwined with the fibers in the sheets when the gypsum slurry is enclosed between the first and second sheets. Preferably, the first and second sheets each have a mean flow pore size, measured according to ASTM F316-86, of at least 8 microns, and more preferably in the range of 8.7 to 40 microns.
According to the more preferred process of the invention, the first and second sheets are nonwoven sheets. Preferably, the first surface of the first sheet and the first surface of the second sheet are made of sheets selected from the group of needle punched staple fiber sheets, hydroentangled fibrous sheets, and spunbond sheets.
Alternatively, the first surface of the first sheet and the first surface of the second surface of the second sheet may be embossed surfaces. According to another alternative embodiment of the invention, the first surface of the first sheet and the first surface of the second sheet are precoated with a layer of a high density gypsum slurry having a density that is at least 1.1 times the density of the gypsum slurry.
The present invention is also directed to a gypsum board, comprising: a first polymeric fibrous sheet having a first surface; a second polymeric fibrous sheet having a first surface; a wet and stiff gypsum core sandwiched between the first and second sheets; wherein the first surface of the first sheet and the first surface of the second sheet have pores containing set gypsum of said gypsum core intertwined with the fibers in the first and second sheets, the first and second sheets adhering to said wet gypsum core with an adhesive strength of at least 7.5 lb. Preferably, the first and second sheets adhere to the wet gypsum core with an adhesive strength of at least 10 lb. It is further preferred that the first and seconds sheets be nonwoven sheets. The first surface of said first sheet and the first surface of the second sheet may be comprised of a web selected from the group of needle punched staple fiber sheets, hydroentangled fibrous sheets, and spunbond sheets. The first surface of the first sheet and the second surface of the second sheet may be embossed surfaces. According to another embodiment of the invention, the first surface of the first nonwoven sheet and the first surface of the second nonwoven sheet are coated with a primer layer of a high density gypsum slurry having a density that is at least 1.1 times the density of the gypsum core.